The present invention relates to integrated circuit fabrication, specifically to plasma etching processes.
Background: Plasma Etching
Integrated circuit fabrication technology has evolved rapidly in recent years. The demand for smaller devices has required features in wafer fabrication to shrink to extremely fine sizes. Plasma etching has grown into one of the more commonly used fabrication processes because of its ability to etch small sizes with controllable selectivity and anisotropy.
Simple plasma reactors consist of parallel plate capacitors in a chamber that can be maintained at low pressure. A high frequency voltage is applied between the electrodes, and current flows which dissociates a gas, ionizing a small number of its molecules to form a plasma. For most etching processes, the extent of ionization is very small, on the order of one particle per 100,000 or 1,000,000. Reactive radicals are produced by the electrical discharge. The positive charge consists mostly of singly ionized neutrals which have lost a single electron. The majority of negative charged particles are free electrons, though in very electronegative gases negative ions can be more abundant.
During etching, semiconductor wafers on the electrode surface are exposed to the reactive neutral and charged species. Some of these species combine with the substrate material to form volatile products that evaporate, etching the substrate while leaving other materials (the mask, for instance) relatively unaffected. Plasma etching can thus selectively remove films while masks and underlying materials are not etched.
Though plasma etching is capable of etching patterns with the necessary resolution, the process must be strictly controlled in order to produce consistently high quality patterns. With the rapid decrease of feature size of semiconductor devices, multi-layer interconnect technology becomes both critical to the success of process and design and challenging technologically. Specifically, processes such as oxide etch should be reliable, have good throughput, and have precisely controllable performance.
Fluorine/carbon based chemistry at low pressure and high density plasma sources are used to produce higher etch rate and higher aspect ratio etch capabilities. In this type of system, F/C ratio is a key factor in etch performance. Etching must last long enough to completely remove the desired material layer, but must not excessively overetch and thus cause damage to underlying structures and materials.
Endpoint detection for etching processes is therefore very important. Etching occurs at optimized and balanced levels, and these balances can indicate when etching has proceeded to materials beyond the intended etch materials. So, for example, when an etchant fully etches an oxide layer, and begins to interact with the nitride beneath, these balances are disturbed. Such changes in the plasma system are used to determine the proper time to cease etching.
Endpoint can be determined in a number of ways. One common method for determining endpoint is through spectral emissions of reactant gases in the plasma chamber. The intensities of spectra emitted by the gases change whenever the electrical and chemical conditions in the system change. Such changes occur when a desired layer is fully etched, exposing the underlying layer. When the etched layer is fully removed, its contribution to the system changes, and its spectrum intensity reflects this change. By monitoring these spectral emissions, an endpoint for etching can be determined. However, for small open areas, such methods decrease in effectiveness because smaller open areas means smaller spectral differences when a particular layer is fully etched. Thus the changes in plasma system conditions are more difficult to detect using small open area and optical emission spectra.
Another method of determining endpoint includes monitoring the voltage across the sheath region of the plasma. The DC bias changes during the resist strip cycle, and reaches a maximum when the etched film begins to clear. This type of monitoring usually requires a probe placed inside the plasma chamber to measure voltage changes. Such probes are not only difficult to include in the process, they alter the process themselves and must be accounted for in the plasma system. They also share a disadvantage with optical systems in that the etching cycle must be completed before the signal for endpoint is generated. This increases the risk of overetch.
Endpoint Control for Small Open Area by RF Source Parameter
The present application discloses monitoring a DC component of the impedance matching network to determine a stopping point for plasma etching. The innovative endpoint detection system operates by monitoring the voltage change across a resistor in the matching network. The voltage drop in the resistor is proportional to the total DC voltage of the plasma system. Thus changes in the plasma DC voltage are indirectly monitored to determine an endpoint for etching.
This indirect method of monitoring the plasma DC parameters allows easier endpoint detection than typical DC bias monitoring methods. Instead of in-situ monitoring via probes, a simple voltmeter added to the matching circuit can indicate etch endpoint. This method works even for small open area percentages and indicates changes in the process earlier than optical emission spectrum endpoint detection schemes.
Advantages of the disclosed methods and structures, in various embodiments, can include one or more of the following:                endpoints for small open areas may be detected;        endpoint signaled before optical emission spectrum signals endpoint;        implementation with minor changes to existing systems.        